CHP 4

• Soil is a construction material used in many structures such as retaining walls, dams, and levees
• Soil is also a foundation material upon which structures rest
• All structures ultimately rest upon soil or rock
• Purpose of engineering investigation:

  • Provides first-hand information for selection of foundation type
  • Helps in the design of foundations
  • Allows contractors to quote realistic and comprehensive tenders
  • Assists in planning construction techniques
  • Aids in the selection of appropriate construction equipment for excavation
  • Helps in estimating development cost for the site
  • Facilitates the study of environmental impacts of the proposed construction
• Soil mechanics is the branch of mechanics that deals with the action of forces on soil masses
• Soil is one of the most widely encountered materials in civil, structural, and architectural engineering
• Soil testing is primarily done to test the bearing capacity of the soil and to show the physical and chemical composition of the soil
• Importance of soil investigation:

  • Understanding geotechnical investigation of the soil helps in making better decisions leading to the success of the construction project
  • Engineers decide the height of the building and the use of materials based on soil testing reports
• Soil Volume & Density Relationship:

  • Density: Mass of a unit volume of soil (M/V): kg/m³
  • Wet density: Soil + water; Dry density: Soil only
  • Moisture content (w): the ratio of the mass of water to the mass of solid soil
  • Porosity (n): amount of water/air contained in the voids
  • Void ratio (e): the ratio of the void volume to the volume of soil particles
  • Saturation degree: the ratio of the water volume to the volume of void
• Permeability/hydraulic conductivity: property of soil allowing it to transmit water
• Soil Consistency: Atterberg Limits

  • Liquid Limit (LL): The water content (%) at which soil begins to behave as a liquid material
  • Plastic Limit (PL): The moisture content (%) at which soil begins to behave as a plastic material
  • Plasticity Index (PI): Numerical difference between LL and PL
  • Liquidity Index
• Soil strength:

  • Shear strength is the maximum shear resistance that materials are capable of developing
  • Shear strength consists of friction between particles and cohesion due to a chemical bond between soil particles
  • Cohesion (C') is a measure of the forces that cement particles of soils
• Soil Strength: Cohesion
• Soil Strength: Cohesion (C’) is a measure of the forces that cement particles of soils
• True cohesion in soils is caused by:

  • Electrostatic forces in stiff over consolidated clays
  • Cementing
• Apparent cohesion is caused by:

  • Negative capillary pressure (lost upon wetting)
  • Pore pressure response during undrained loading (lost through time)
  • Root cohesion (may be lost through logging or fire of the contributing plants, or through solution)
• Soil Strength: Internal Friction Angle (ϕ’)
• The shear strength of a granular soil (sands, gravel & some silts) is closely related to the friction resistance of solids in contact
• Internal friction angle (ϕ’) is a measure of the shear strength of soils due to friction
• DIRECT SHEAR TEST: Inexpensive, fast and simple especially for granular soils
• DIRECT SHEAR STRESS : Reliability of the results may be questioned because the soil is not allowed to fail along the weakest plane, but is forced to fail along the plane of split of the shear box
• DIRECT SHEAR STRESS: Shear stress distribution over the shear surface of the specimen is not uniform (Area of sliding surface changes as the test progresses)
• Due to smaller thickness of the sample, rapid drainage can be achieved
• Large deformation can be achieved by reversing shear direction, useful for determining the residual strength of soil
• Developed by Casagrande in an attempt to overcome some of the serious disadvantages of the direct shear test
• In a triaxial shear test, stress is applied to a sample of the material being tested in a way which results in stresses along one axis being different from the stresses in perpendicular directions
• [TRIAXIAL SHEAR STRESS] Advantages:

  • More versatile
  • Drainage can be well controlled
  • No rotation of the principal stresses like the direct shear test
  • The failure plane can occur anywhere
• Hydraulic conductivity is a property of soils or rocks that describes the ease with which a fluid (usually water) can move through pore spaces or fractures
• Saturated hydraulic conductivity, Ksat, describes water movement through saturated media
• Two broad categories of determining hydraulic conductivity:

  • Empirical approach correlated to soil properties like pore size and particle size distributions, and soil texture
  • Experimental approach determined from hydraulic experiments using Darcy's law
• Cohesive soil means clay (fine grained soil), or soil with a high clay content, which has cohesive strength
• Cohesive soil is hard to break up when dry, and exhibits significant cohesion when submerged
• Cohesive soils include clayey silt, sandy clay, silty clay, clay and organic clay
• Non-cohesive soil do not have cohesive forces and are governed by self-weight
• Cohesive soil has internal friction and significant cohesion when submerged
• Sand is a typical example of non-cohesive soil
• Two standard laboratory tests used to determine the hydraulic conductivity of soil:

  • The constant-head test
  • The falling-head test
• Factors causing reduction in soil strength:

  • Vibration and earthquakes
  • Increase in moisture content
  • Increase in pore water pressure
  • Loss of cementing material
• Factors controlling shear strength of soils:

  • Soil compaction (Grain size distribution, shape of particles)
  • State (Loose, dense, stiff, soft, etc.)
  • Structure (Disturbed, undisturbed, compacted, cemented, etc.)
  • Loading conditions (Drained, undrained, loading, etc)
• IMPORTANCE OF SOIL STRENGTH

  1. Shear Failure in soils
  2. Bearing Capacity failure